The present invention relates to an apparatus for processing halftone image, which is applicable for estimating a halftone image from a binary coded image, for instance, the apparatus can satisfactorily estimate an original binary coded image based on a binary coded image expressed in halftone.
Most of output apparatuses currently subjected to practical use, such as display apparatus and printing apparatus, can express an image using two values of black and white.
As a method for obtaining a pseudo halftone image using these apparatuses, the density pattern method (brightness pattern method) or dither method are well-known.
The density pattern method and the dither method are one kind of the tone production method by density of each element, which obtains a halftone image by changing the number of dots recorded within a specific area (matrix).
The density pattern method records an area corresponding to one pixel of an original image with a plurality of dots using a threshold value matrix as shown in FIG. 33-b. The dither method records an area corresponding to one pixel of an original image with one dot using a threshold value matrix as shown in FIG. 33-a. Both methods can obtain binary coded output data, which express a pseudo halftone image of the original using two codes of black and white.
If the original halftone image (corresponding to the input data shown in FIG. 33) can be estimated from the pseudo halftone binary coded image thus-obtained, various types of data processings are enabled, thereby the degree of freedom in image transformation can be advantageously increased.
The density pattern image can be immediately returned to its original halftone image if the used pattern level configuration can be obtained. The resolution, however, is low for the information volume.
On the contrary, resolution of dither images is higher than that of density pattern images as compared with its information volume. However, it is difficult to return the dither image to its original halftone image. Therefore, various types of image transformation cannot be achieved using only the dither image.
Even when a multiple-coded image having three or more levels is used as an original image, the situation is the same as mentioned above.